linux-kernel - Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling

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Date: Thu, 15 Feb 2024 13:24:59 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: paulmck@...nel.org
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, peterz@...radead.org,
        torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
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        David.Laight@...lab.com, richard@....at, jon.grimm@....com,
        bharata@....com, boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling


Paul E. McKenney <paulmck@...nel.org> writes:

> On Wed, Feb 14, 2024 at 07:45:18PM -0800, Paul E. McKenney wrote:
>> On Wed, Feb 14, 2024 at 06:03:28PM -0800, Ankur Arora wrote:
>> >
>> > Paul E. McKenney <paulmck@...nel.org> writes:
>> >
>> > > On Mon, Feb 12, 2024 at 09:55:24PM -0800, Ankur Arora wrote:
>> > >> Hi,
>> > >>
>> > >> This series adds a new scheduling model PREEMPT_AUTO, which like
>> > >> PREEMPT_DYNAMIC allows dynamic switching between a none/voluntary/full
>> > >> preemption model. However, unlike PREEMPT_DYNAMIC, it doesn't depend
>> > >> on explicit preemption points for the voluntary models.
>> > >>
>> > >> The series is based on Thomas' original proposal which he outlined
>> > >> in [1], [2] and in his PoC [3].
>> > >>
>> > >> An earlier RFC version is at [4].
>> > >
>> > > This uncovered a couple of latent bugs in RCU due to its having been
>> > > a good long time since anyone built a !SMP preemptible kernel with
>> > > non-preemptible RCU.  I have a couple of fixes queued on -rcu [1], most
>> > > likely for the merge window after next, but let me know if you need
>> > > them sooner.
>> >
>> > Thanks. As you can probably tell, I skipped out on !SMP in my testing.
>> > But, the attached diff should tide me over until the fixes are in.
>>
>> That was indeed my guess.  ;-)
>>
>> > > I am also seeing OOM conditions during rcutorture testing of callback
>> > > flooding, but I am still looking into this.
>> >
>> > That's on the PREEMPT_AUTO && PREEMPT_VOLUNTARY configuration?
>>
>> On two of the PREEMPT_AUTO && PREEMPT_NONE configurations, but only on
>> two of them thus far.  I am running a longer test to see if this might
>> be just luck.  If not, I look to see what rcutorture scenarios TREE10
>> and TRACE01 have in common.
>
> And still TRACE01 and TREE10 are hitting OOMs, still not seeing what
> sets them apart.  I also hit a grace-period hang in TREE04, which does
> CONFIG_PREEMPT_VOLUNTARY=y along with CONFIG_PREEMPT_AUTO=y.  Something
> to dig into more.

So, the only PREEMPT_VOLUNTARY=y configuration is TREE04. I wonder
if you would continue to hit the TREE04 hang with CONFIG_PREEMTP_NONE=y
as well?
(Just in the interest of minimizing configurations.)

---
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE04 b/tools/testing/selftests/rcutorture/configs/rcu/TREE04
index 9ef845d54fa4..819cff9113d8 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE04
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE04
@@ -1,7 +1,7 @@
 CONFIG_SMP=y
 CONFIG_NR_CPUS=8
-CONFIG_PREEMPT_NONE=n
-CONFIG_PREEMPT_VOLUNTARY=y
+CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n